Lauren Faust recently dropped some backstory on Spike over on her Deviant Art page. Apparently he was actually raised by Celestia, which makes a whole lot more sense than Twilight doing it.
This isn't 100% canon, but it's more information on the Spike/Twilight relationship than we have to go off of right now. Hopefully all of this will be explored in the actual show as we get more episodes!
Thanks to YazzoB for the heads up!